This is not a comprehensive list by any means, and is why I'm featuring this post and leaving it open to comments. I may update this as people make suggestions, and I encourage others to make their own variations on this list in the comments or in their respective versions of this community.

Also to clarify some terms here, by active I'm aiming for at least some commenting on recent-ish posts, not merely recent posts, and by the topic names...I'll let the linked communities serve as examples.

Hope this helps any newcomers find some different communities to join and participate in!

p.s. also keep an eye on this post as a useful resource for a broader range of communities than those listed here, and recently, more regularly updated.